Arbuscular Mycorrhizal Fungi Promote Nodulation and N₂ Fixation in Soybean by Specific Root Exudates

Legume plants commonly associate with both arbuscular mycorrhizal (AM) fungi and rhizobia and thus enhance the acquisition of phosphorus (P) and nitrogen (N) nutrition. Inoculation with AM fungi can promote nodulation and N₂ fixation of legume plants; however, the underlying mechanisms remain poorly understood. Here, root exudates collected from AM-colonised soybean plants showed greater accumulation of the specific flavonoids (daidzein and genistein) and phenolic acids (benzoic acid and p-Hydroxybenzoic acid), and significantly promoted nodulation. Furthermore, the exudates from AM-colonised roots and the derived specific flavonoids and phenolic acids effectively increased rhizobial growth, chemotaxis, biofilm formation. Addition of the specific synthetic root exudates enhanced nodulation and N₂ fixation, and expression of the core nodulation genes in soybean. Overexpression of a phenylalanine ammonia-lyase gene, GmPAL2.4 markedly upregulated the expression of the genes related to the biosynthesis of daidzein, genistein, benzoic acid, and p-Hydroxybenzoic acid, and increased accumulation of these specific flavonoids and phenolic acids in the transgenic plants, thus enhancing nodulation and N₂ fixation. In summary, we demonstrated a crucial role of specific flavonoids and phenolic acids induced by AM symbiosis in promoting rhizobium-host symbiosis. This offers a pathway for improving symbiotic efficiency through the use of specific synthetic compounds.